Vibrating meters such as, for example, densitometers, volumetric flow meters, and Coriolis flow meters are used for measuring one or more characteristics of substances, such as, for example, density, mass flow rate, volume flow rate, totalized mass flow, temperature, and other information. Vibrating meters include one or more conduits, which may have a variety of shapes, such as, for example, straight, U-shaped, or irregular configurations.
The one or more conduits have a set of natural vibration modes, including, for example, simple bending, torsional, radial, and coupled modes. The one or more conduits are vibrated by at least one driver at a resonant frequency in one of these modes, hereinafter referred to as the drive mode, for purposes of determining a characteristic of the substance. One or more meter electronics transmit a sinusoidal driver signal to the at least one driver, which is typically a magnet/coil combination, with the magnet typically being affixed to the conduit and the coil being affixed to a mounting structure or to another conduit. The driver signal causes the driver to vibrate the one or more conduits at the drive frequency in the drive mode. For example, the driver signal may be a periodic electrical current transmitted to the coil.
One or more pick-offs detect the motion of the conduit(s) and generate a pick-off signal representative of the motion of the vibrating conduit(s). The pick-off is typically a magnet/coil combination, with the magnet typically being affixed to one conduit and the coil being affixed to a mounting structure or to another conduit. The pick-off signal is transmitted to the one or more electronics; and according to well-known principles, the pick-off signal may be used by the one or more electronics to determine a characteristic of the substance or to adjust the driver signal, if necessary.
Typically, in addition to the conduits, vibrating meters are also provided with a case. The case can protect the conduits from the environment as well as provide secondary containment in the event of a conduit failure. The vibrational characteristics of the case can cause significant measurement problems. The measurement problems caused by the case are due to the difficulty in differentiating vibrations associated with the conduits from vibrations associated with the meter's case. One reason for the difficulty is that similar to the conduits, the case also has one or more natural modes of vibration, including for example, simple bending, torsional, radial, and lateral modes. The particular frequency that induces a mode of vibration generally depends on a number of factors such as the material used to form the case, the thickness of the case, the shape of the case, temperature, pressure, etc. Vibrational forces generated by the driver or from other sources in the material processing system, such as pumps, may cause the case to vibrate in one of the natural modes. It is difficult to generate an accurate measurement of a characteristic of the substance in situations where the frequency used to drive the one or more conduits in the drive mode corresponds to a frequency that causes the case to vibrate in one of its natural modes of vibration. The vibrational modes of the case can interfere with the vibration of the conduits leading to erroneous measurements.
One reason for vibrational interference of the case is due to the relatively large side panels of some cases. Such a case is shown in FIG. 1.
FIG. 1 shows a prior art case 100 for a vibrating meter. The case 100 includes a first case portion 100a and a second case portion 100b. In use, the case 100 can surround other components of the vibrating meter (See FIGS. 3 & 4). The case 100 and more particularly, the first case portion 100a, comprises a first panel 101. The first panel 101 is generally flat, but may include some curvature. The first panel 101 extends from a first edge 102, which is shown at the bottom of the case 100 as shown in the figure, to a second edge 103 at the top of the case. The first panel 101 also extends from a third edge 104 on a first end of the case 100 to a fourth edge 105 at a second end of the case 100. The edges 102, 103, 104, and 105 can define an outer boundary of the first panel 101. The first edge 102 comprises the end of the first case portion 100a and provides a boundary for the second case portion 100b to be coupled to. The second 103, third 104, and fourth 105 edges create the boundary between the panel 101 and a transition section 106. The transition section 106 comprises a curved region that joins opposing panels 101 (the panel visible in FIG. 1 and the other panel on the opposite side of the case 100, which is not visible in FIG. 1). The transition section 106 can generally define the overall width, w, of the case 100. According to an embodiment, the edges 102, 103, 104, 105 can substantially change the direction of the case 100, for example. As can be seen, the panel 101 is generally flat; however, upon reaching an edge 102, 103, 104, or 105, the direction substantially changes and is no longer flat, thereby creating a boundary and end of the panel 101. As can be appreciated, the opposite side of the case 100 looks substantially the same and includes the same components and thus, is not shown for brevity of the description.
As can be appreciated, the panel 101 is relatively large with respect to the overall size of the case 100 and for a given material and thickness can have a relatively low resonant frequency. In some embodiments, the relatively low resonant frequency can overlap with the intended drive frequency of the conduits contained within the case 100. This overlap can create measurement problems in vibrating meters.
There have been numerous prior art attempts to separate the frequencies that induce the case's vibrational modes from the conduits' vibrational modes. These frequencies may comprise the natural resonant frequencies of the various vibrational modes of the case and the fluid filled conduits. For example, the case can be made extremely stiff and/or massive in order to separate the frequencies that induce the various vibrational modes away from the anticipated drive mode of the conduits. Both of these options have serious drawbacks. Increasing the mass and/or stiffness of the case results in complex and difficult manufacturing, this adds cost and makes mounting the vibrating meter difficult. One specific prior art approach to increasing the mass of the case has been to weld metal weights to an existing case. This approach does not adequately dissipate vibrational energy in order to separate the case's resonant frequencies well away from the intended drive frequency. Further, this approach is often costly and produces an unsightly case.
Another prior art approach to separating the frequencies that induce the case's vibrational modes from the intended drive frequency has been to add ribs to the case. The ribs are either coupled to the case or formed as part of the case. Such an approach can be seen in FIG. 2 showing a prior art case 200. The ribs 204 extend along a small portion of the case's panel 101 in an attempt to increase the case's resonant frequencies in an attempt to exceed the intended drive frequency. However, the ribs often fail to provide complete frequency separation. Therefore, there exists a need in the art for a case having vibrational characteristics that are adequately separated from the intended drive frequency of the conduits without suffering from the above-mentioned drawbacks. The embodiments described below overcome these and other problems and an advance in the art is made. The embodiments described below comprise a case including one or more indentations formed in one or both of the case's panels and extending completely from a first panel edge to a second panel edge. By extending the indentations completely between two panel edges, the case's resonant frequencies are increased.